It is necessary to fill the railing by using figures from 1 to 9. Necessarily only once in every line, column and square of 3 x 3.
To resume the vocabulary of the guide
It is necessary to fill the railing by using 9 given symbols (Generally figures from 1 to 9, or letters indicated at the foot of the railing). Necessarily only once (but no more) on every line, column and region (or squared by 3 x 3).